It was never an easy relationship. For a while, though, the Defense of Marriage Act and the Obama administration managed to live together. Then the administration walked out, leaving DOMA to fend for herself.

There’s no point asking why. We know why -- the president’s core supporters refuse to recognize what centuries of tradition and common sense tell us, and what voters and lawmakers confirmed in 1996 -- that marriage is between one man and one woman. The question is, what’s next for this act scorned?

Congress shouldn’t passively accept this challenge to its authority. Apart from anyone’s personal views on marriage is this unavoidable fact: The executive branch of our government (the president) is obligated, under the Constitution, to enforce the laws passed by the legislative branch (Congress). And whether the administration likes it or not, DOMA is the law of the land.

Fortunately, House Republican leaders late last week signaled their intention to defend DOMA. “This regrettable decision was based on a standard no court has yet found to apply and is a clear political exercise by the administration,” Majority Leader Eric Cantor, R-Va., said.

Indeed it is. DOMA passed both houses of Congress with overwhelming majorities and was signed by President Clinton. Nearly 40 states have enacted their own versions, and traditional marriage has been enshrined in 31 state constitutions. Its validity and constitutionality are beyond question.

Yet the administration, by walking away from DOMA, is rejecting this. In effect, it’s saying, “Congress, you think the laws you pass on behalf of your constituents deserve a defense in court? Not unless we agree with them. Voters, you think marriage should be reserved for one man and one woman? You’re bigoted and irrational.”

It isn’t just this particular law that’s at stake. If Congress fails to ensure that DOMA receives a robust defense, it will have failed to defend its lawmaking authority. Lawmakers need to stand up for their prerogatives as legislators. Otherwise, it’s no exaggeration to say we risk undermining our whole system of government.

It’s not hard to see why. Are we a nation of laws, or can we pick and choose between those laws we happen to agree with at a particular time, and those we don’t? What laws will be on the executive branch chopping block tomorrow?

Consider the words of former U.S. Attorney General Benjamin Civiletti, who served during the last two years of the Carter administration. In a 1980 letter to the Senate regarding his duty to enforce the law, Civiletti wrote: “If executive officers were to adopt a policy of ignoring or attacking Acts of Congress whenever they believed them to be in conflict with the provisions of the Constitution, their conduct in office could jeopardize the equilibrium established within our constitutional system.”

And if an attorney general finds himself defending a law he disagrees with? Then, according to Civiletti, “it is almost always the case that [the Attorney General] can best discharge the responsibilities of his office by defending and enforcing the Act of Congress.” That’s how the Bush administration handled the Bipartisan Campaign Reform Act. The president said that certain provisions of the act were likely unconstitutional, but his Justice Department mounted a vigorous defense of the legislation in court.

In doing so, the Bush administration upheld a long-standing principle -- to defend a federal statute unless no reasonable argument can be made for it, or unless the statute would infringe on some core presidential authority. Neither applies in DOMA’s case. Yet the administration abandoned it.

In a way, it’s for the best. DOMA deserves better than the half-hearted defense it has been getting so far. Now Congress can finally put the case in the hands of legal counsel who can ensure that the law of the land gets the full-throated support it should have had all along.